1. Field of the Invention
The invention relates to methods for joining or repair of metal components, including advanced superalloy components, by resistance brazing. More particularly, the present invention methods utilize high-resistivity braze fillers that melt with relatively low heat input by application of electric current, without impacting structural properties of the underlying substrate metal. In some embodiments, the invention relates to methods for surface repair of defects in superalloy turbine blades and vanes in steam or gas turbines by filling the defects with a high resistivity braze alloy. Other embodiments relate generally to filling of surface defects in metal substrates or joining of two substrates in fabrication or repair of metal components, especially superalloy metal components, with a high resistivity braze alloy.
2. Description of the Prior Art
Repair or new fabrication of nickel and cobalt based superalloy material that is used to manufacture turbine components, such as cast turbine blades, is challenging, due to the metallurgic properties of the finished blade material. For example a superalloy having more than 6% aggregate aluminum or titanium content, such as CM247 alloy, is more susceptible to strain age cracking when subjected to high-temperature welding than a lower aluminum-titanium content X-750 superalloy. The finished turbine blade alloys are typically strengthened during post casting heat treatments, which render them difficult to perform subsequent structural welding. Currently used welding processes for superalloy fabrication or repair generally involve substantial melting of the substrate adjoining the weld preparation, and complete melting of the welding filler material added. When a blade constructed of such a material is welded with filler of the same or similar alloy, the blade is susceptible to solidification (aka liquation) cracking within and proximate to the weld, and/or strain age (aka reheat) cracking during subsequent heat treatment processes intended to restore the superalloy original strength and other material properties comparable to a new component.
In the past, electric resistance brazing has been commonly used for joining of common ferrous and non-ferrous (e.g., copper) alloy substrate components that are not superalloys. See for example U.S. Pat. No. 4,924,054. Solid sheet, powder or paste brazing alloy is interposed between the components. Resistance brazing is performed by passing current between compressed electrodes into the pair of abutting substrate components and melting the brazing alloy. Electrodes are often constructed of high resistance material such as carbon, tungsten or molybdenum. Most of the heat generated by the current originates in the electrodes, and that heat is in turn conducted through the joined metal substrate components. As electric current is passed between the electrodes and conducts heat through the substrate components the brazing alloy melts and by capillary action wets and affixes the components to each other.
While known electric resistance brazing methods have been utilized for joining common non-ferrous and ferrous alloys, they have shortcomings for application to the joining or repair of superalloy components. Resistance brazing requires high conduction heat input to the substrate in order to melt the brazing alloy. As noted above high heat application to superalloy materials negatively impacts their structural properties. Liquefied braze alloy must not contact the resistance electrodes, or else the electrode material becomes contaminated. If liquefied braze alloy is interposed between an electrode and the substrate material they may become permanently attached, ruining the electrode and possibly damaging the substrate component. Liquified braze overrun contact with electrodes is more likely when repairing surface cracks that are spread over a relatively wide surface area, such as during repair of superalloy turbine blades and vanes.
Methods for joining superalloy components by respectively electric resistance brazing and electric resistance welding are disclosed and claimed in commonly owned co-pending U.S. Utility patent application Ser. No. 13/352,475, entitled “Projection Resistance Brazing Of Superalloys”; and Ser. No. 13/352,468 entitled “Projection Resistance Welding Of Superalloys” both filed on Jan. 18, 2012. More specifically, that co-pending applications disclose methods for joining superalloy components along a contact surface between a mating projection and recess formed in each respective component. The components are compressed and resistance heated along their common contact surface until the brazing material melts (when brazing) or the contact surfaces liquefy or plasticize (when welding), which joins the opposing contact surfaces. In these applications it is disclosed that the resistance heat is concentrated along the mating contact surfaces and does not impact the underlying structural properties of the superalloy components. The applications additionally disclose and claim that a surface defect in a superalloy component can be repaired by excising the surface defect and preparing a superalloy splice having a surface projection profile that conforms to the excised material surface profile. The superalloy splice fills the space formerly occupied by the excised material and is joined to the repaired superalloy substrate by the disclosed resistance brazing or welding method. Thus the repaired superalloy component is effectively reconstructed with a new superalloy splice having the same or similar structural properties as the repaired component. However, for some superalloy repairs where hid structural strength is not necessary, it is preferable to avoid the need to excise defective material and fabricate a complementary filler splice.
Non-structural repair or fabrication of metals, including superalloys, is recognized as replacing damaged material (or joining two components of newly fabricated material) with mismatched alloy material of lesser structural property specifications, where the localized original structural performance of the original substrate material is not needed. For example, non-structural or cosmetic repair may be used in order to restore the repaired component's original profile geometry. In the gas turbine repair field an example of cosmetic repair is for filling surface pits, cracks or other voids on a turbine blade airfoil in order to restore its original aerodynamic profile, where the blade's localized exterior surface is not critical for structural integrity of the entire blade. Cosmetic repair or fabrication is often achieved by using oxidation resistant weld or braze alloys of lower strength than the blade body superalloy substrate, but having higher ductility and lower application temperature that does not negatively impact the superalloy substrate's material properties.
Diffusion brazing has been utilized to join superalloy components for repair or fabrication by interposing brazing alloy between their abutting surfaces to be joined and heating those components in a furnace (often isolated from ambient air under vacuum or within an inert atmosphere) until the brazing alloy liquefies and diffuses within the substrate of the now-conjoined components. Diffusion brazing can also be used to fill surface defects, such as cracks, in superalloy components by inserting brazing alloy into the defect and heating the component in a furnace to liquefy the brazing alloy and thus fill the crack. In some types of repairs a torch, rather than a furnace can be used as a localized heat source to melt the brazing alloy. When performing diffusion or torch brazing repairs of surface defects, unlike known electric resistance brazing methods, any liquefied brazing alloy overruns out of cracks does not cause potential damage to electrodes or inadvertent attachment of electrodes to the repaired substrate.
When performing diffusion or torch brazing on superalloy components care must be taken to avoid overheating the substrate and causing its structural degradation, as discussed above. To this end, brazing alloys with relatively low melting points have been used to minimize heating of the overall superalloy substrate. U.S. Pat. No. 7,156,280 states that nickel- or chromium-based high-temperature braze alloy compositions used to fill wide gaps in superalloy component diffusion brazing repair can include chromium (Cr), hafnium (Hf) and/or boron (B) to suppress the braze alloy's melting point, so as to reduce likelihood of superalloy degradation. It is also known that silicon (Si) and phosphorus (P) also suppress the melting point of nickel alloy brazes. Thus, B, Si and P are constituents in commercially available powdered and solid brazing alloys that are recommended for low melting temperature diffusion and torch brazing applications.
Previously developed superalloy repair methods all have various favorable and less favorable attributes. The electric resistance brazing and welding methods described and claimed in the commonly owned co-pending patent applications, by removal of damaged material and replacement with a splice of new material, provide for high-quality structural repairs with relatively simple repair apparatus and methods, but may not be considered as commercially cost effective for relatively simple cosmetic surface repairs on superalloy components, such as turbine blades and vanes. Generally diffusion brazing processes require relatively long thermal cycling times, complicated metal treatment processes with relatively expensive treatment equipment and relative slow through put times for service repair as compared to known torch or electric resistance brazing techniques. Torch brazing requires significant hand labor that increases costs, slows repair time and introduces the potential for quality control variations dependent upon the skill level of an individual metal worker who is performing the repair. Other commonly known cosmetic repair electric resistance brazing methods are not suitable for repairing relatively wide or extensive cracks due to potential electrode damage and/or inadvertent substrate attachment caused by liquefied brazing alloy overruns out of the cracks.
Thus, a need exists in the art for a method for performing joining of or repairs on surfaces of metallic components, including superalloy components such as turbine vanes and blades, so that subcomponents can be joined or that cracks and other surface defects can be repaired, without degrading structural properties of the component substrate.
Another need exists in the art for a method for performing joining of or repairs on surfaces of metallic components, including superalloy components such as turbine vanes and blades, with proven, repeatable repair techniques and repair equipment that do not require complex welding or post-repair heat treatment procedures that might also degrade structural properties of the component substrate.
Yet another need exists in the art for a method for performing joining of or repairs on surfaces of metallic components, including superalloy components such as turbine vanes and blades, with minimized hand labor and relatively short repair cycle times.